The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins are generally used as the encapsulating resin because they have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions.
In the epoxy resin compositions, curing catalysts are often blended in order to enhance the reactivity of epoxy resins with curing agents. Phosphorus compounds, imidazole compounds, and amine compounds are well known as the curing catalysts. Of these, phosphorus compounds are commonly used because they afford cured products which are highly reliable in terms of moisture resistance or the like, as compared with the imidazole and amine compounds.
To comply with the recent demand for electronic equipment of reduced size and weight, the trend is to reduce the outer dimensions, especially thickness of packages. In order that resin compositions be molded around such thin packages, the resin compositions must have a sufficiently low viscosity and be effectively curable and mold releasable.
Of the phosphorus curing catalysts, triphenyl-phosphine and tetraphenylphosphonium tetraphenylborate are well known and commonly used. Because of low catalytic activity, triphenylphosphine must be blended in epoxy resin compositions in so large amounts that when molded, the resulting high viscosity compositions tend to give rise to troubles such as wire flow, pad shift, void formation, and short molding. Tetraphenylphosphonium tetraphenylborate has lower catalytic activity than triphenylphosphine and is thus difficult to endow resin compositions with sufficient curability even when blended in considerably large amounts. The curing catalysts blended in large amounts give rise to a problem that reaction can take place even at temperatures near room temperature. The resin compositions accordingly increase their viscosity and become less stable during shelf storage.
JP-B 56-45491 discloses an epoxy resin composition comprising an epoxy resin and a curing agent, having good shelf stability and affording highly moisture resistant cured products. The curing agent is obtained by heat treating a mixture of a novolac type phenolic resin and tetraphenylphosphonium tetraphenylborate at a temperature above the softening point of the phenolic resin until the resin is colored slightly yellowish brown or dark brown. However, since this curing catalyst yet has low activity and lacks fast curing ability, it must be used in a large amount at the sacrifice of the shelf stability of the epoxy resin composition.
In JP-A 7-242683 corresponding to U.S. Pat. No. 5,473,091, the same assignee as the present invention proposed an epoxy resin composition having blended therein a curing catalyst which is obtained by mixing for reaction a quaternary phosphorus compound with a compound having phenolic hydroxyl groups at a temperature of 120 to 250.degree. C. This epoxy resin composition is still insufficient with respect to low viscosity, good cure, smooth mold release, and shelf stability.